AMPK (AMP-activated protein kinases) is an enzyme that functions to regulate a metabolic pathway so as to maintain balance between supply of nutrients and demand for energy, and thus maintain energy homeostasis in cells and the whole body. AMPK is activated as a ratio of AMP/ATP in the cells increases due to a hypoxemic state or glucose deficiency. The activated AMPK induces fatty acid oxidation to produce a larger amount of ATP and inhibits anabolisms requiring the use of ATP. AMPK inhibits proliferation of cancer cells and kills the cancer cells by regulating energy metabolism in the cancer cells as well as in normal cells. AMPK activated in the cancer cells shows anticancer activities by phosphorylating mTORC1, p53, fatty acid synthase and the like to regulate the cell cycle, cell polarity, autophagy, apoptosis, etc.
Metformin has been used to treat insulin-independent diabetes mellitus (i.e., type II diabetes mellitus) since, among oral therapeutic agents for treating diabetes mellitus, it is most effective at lowering blood glucose, does not cause hypoglycemia or hyperinsulinemia and can prevent complications. In recent years, metformin has been extensively researched. Also, it was reported that metformin activates AMP-activated protein kinase (AMPK) by inhibiting the action of complex 1 of the electron transport system in mitochondria to obstruct intracellular generation of energy, and inhibits activation of the mTOR/S6K1 signaling pathway in which proteins essential for survival are produced to obstruct proliferation of cancer cells and tumor growth (Mol. Cancer Ther. 9(5): 1092-1099 (2010)). Consequently, metformin has received considerable attention as an anticancer agent for regulating cancer cell metabolism. Also, an epidemiological survey confirmed that the incidence of cancer and mortality by cancer were lower for patients treated with metformin (BMJ.330: 1304-1305 (2005)).
Meanwhile, there is increasing clinical evidence indicating that cancer stem cells take part in recurrence and metastasis of cancer. The cancer stem cells refer to cancer cells that have self-regeneration or differentiation activities which are characteristically innate to stem cells. The cancer stem cells are present in the cancer tissues at a content of 0.2% or less, and are characterized by their slow proliferation. Since many anticancer agents developed so far target cancer cells that proliferate rapidly, the cancer stem cells are resistant to conventional anticancer therapy when cancer stem cells are treated with the anticancer agents, thereby causing poor prognoses. On the other hand, it was reported that metformin prevents the recurrence of cancer as it selectively acts only on cancer stem cells among breast cancer cells and removes the cancer stem cells (Cancer Res. 69(19): 7507-11 (2009)). Also, it was found that metformin prevents the metastasis of cancer by interfering with the motility and invasion of the cancer since it inhibits the expression of Snail1, Slug, Twist, ZEB1/2 and TGF-b, which are transcription factors associated with the epithelial-to-mesenchymal transition (EMT) and promotes the expression of E-cadherin to prevent cancer cells from leading to the EMT (Cell Cycle 10: 7, 1144-1151 (2011), Cell Cycle 9: 18, 3807-3814 (2010), Cell Cycle 9: 22, 4461-4468 (2010)).
For these reasons, there is need for a biguanide-based substance that exhibits better pharmacological action than conventional metformin and has improved physiochemical properties.